US3562445A

US3562445A - Preamplifier for high fidelity system utilizing a moving coil stereophonic pickup cartridge

Info

Publication number: US3562445A
Application number: US761683A
Authority: US
Inventors: Neil R Mccanney
Original assignee: PITS MUSIC CO
Current assignee: PITS MUSIC CO
Priority date: 1968-09-23
Filing date: 1968-09-23
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09

Abstract

A preamplifier for a high fidelity system, which utilizes a moving coil, stereophonic, pickup cartridge, provides dynamic damping of the moving coils of that stereophonic, pickup cartridge by interposing an essentially resistive, low impedance, electrical interface between that cartridge and input of that preamplifier. Also, that preamplifier provides an overall gain which is greater than the desired overall gain, so an essentially resistive interface can be used between the output of that preamplifier and the input of the amplifier to which that preamplifier is connected.

Description

United States Patent inventor Neil R. McCanney [56] References Cited S e s" UNITED STATES PATENTS Q 2 33 1968 3,079,463 2/1963 Feldman 179/1 Patented Feb, 1971 3,213,198 10/1965 Claras 179/1- Assignee Pits Music C0.,d.b.a. Best Sound Company OTHER REFERENCES St. Louis, Mo. Tremaine, H.M. The Audio Cyclopedia H.W. Sams & acorporation Co. N.Y. First Edition April 1939 Pages 104,105,341 & 343.

Primary Examiner-Bernard Konick I Assistant Examiner Raymond F. Cardillo, Jr. Attme vKingsland, Rogers, Ezell, Eilers and Robbins ABSTRACT: A preamplifier for a high fidelity system, which PICK UP CARTRIDGE utilizes a moving coil, stereophonic, pick-up cartridge, pro- 7 Cl 4D Fi vides dynamic damping of the moving coils of that stereo-- m mg phonic, pick-up cartridge by interposing an essentially U.S.Cl l79/100.4, resistive, low impedance, electrical interface between that 179/ 1 cartridge and input of that preamplifier. Also, that preamplifi- Int. Cl Gllb 3/00, er provides an overall gain which is greater than the desired H03f 13/00 overall gain, so an essentially resistive interface can be used Field ofSearch 179/1A, between the output of that preamplifier and the input of the 100.4, 100.4ST, 1B, 100.41ST, 180.4Ed, 1.3 amplifier to which that preamplifier is connected.

W 78 J2 '6 13 6'3 22 WV J0 I 86 I [a I 47 I ma fi l 20 i /00 I 7 /02 2f 76 7a /9# [50 /X? /J' J /6 /z x wf 8 'M/( 6 38 PATENTEU FEB 9 I97! I l a 1 I l 20 50 I00 200 500 1000 2000 5000 /0000 20000 CPS l I I 20 5 0 I00 200 500 /000 2000 5000 /0000 20000 PREAMPLIFIER FOR HIGH FIDELITY SYSTEM UTILIZING A MOVING COIL STEREOPI-IONIC PICKUP. CARTRIDGE This invention relates to improvements in Preamplifiers. More particularly, this invention relates to improvements in preamplifiers for high fidelity systems which utilizes a moving coil, stereophonic pickup cartridges.

It is, therefore, an object of the present invention to provide an improved preamplifier for a high fidelity system which utilizes'a moving coil, stereophonic, pickup cartridge.

The use of moving coil, stereophonic, pickup cartridges in high fidelity systems is desirable because such cartridges exhibit a high degree of compliance, and thus minimize the wearing of the microgrooves of the stereophonic discs with which they are used. However, the high degree of compliance of such cartridges reduces the trackability of those cartridges, because it can permit the modulated walls of the microgroove of a stereophonic disc, which was recorded at high intensity levels, to develop sufficient momentum in the stylus and moving coils of such a cartridge to cause that stylusto overshoot its intended path of travel and thereby cause the moving coils of that cartridge to produce an electrical output which differs somewhat from the electrical output which those moving coils should produce. The deviation between the electrical output which the moving coils of that cartridge should produce and the electrical output which those coils actually produce as the stylus overshoots is referred to as transient distortion. A step-up transformer that is presently being marketed for use with moving coil stereophonic pickup cartridges is unable to prevent the transient distortion which occurs when the modulated walls of the microgroove in a stereophonic disc cause the stylus to overshoot. It would be desirable to provide a preamplifier, for use with a moving coil, 7

therefore, an object of the present invention .to provide a preamplifier, for use with a moving coil, stereophonic, pickup cartridge, which will enable the stylus of that cartridge to faithfully follow the modulations on the walls of the microgroove of a stereophonic disc even where that disc was recorded at relatively high intensity.

The preamplifier provided by the present invention makes it possible for the stylus of a moving coil, stereophonic, pickup cartridge to faithfully follow the modulations on the walls of the microgroove of a stereophonic disc, even where that disc was recorded at relatively high intensity levels because that preamplifier provides dynamic damping of the moving coils of that cartridge. That dynamic damping is essentially uniform throughout the range of frequencies to which the moving coils of the moving coil, stereophonic, pickup cartridge must respond, and hence the preamplifier provided by the present invention can enable the stylus of that cartridge to faithfully follow the modulations on the walls of the microgroove of a stereophonic disc, even where that disc was recorded at many different frequencies and at many different intensity levels. The preamplifier of the present invention is. able to provide this desirable dynamic damping because it interposes an essentially resistive electrical interface between each moving coil of the moving coil, stereophonic, pickup cartridge and the corresponding section of that preamplifier, and because it makes the impedance of the electrical interface smaller than the impedance of the moving coil of to which it is connected. As a result, each moving coil of the moving coil, stereophonic, pickup cartridge will coact with the electrical interface to which it is connected to form a closed loop, transducer-load system wherein the impedance of the load is less than the impedance of the transducer: and that closed loop, transducerload system will provide the desired dynamic damping of that moving coil. It is, therefore, an object of the present invention to provide a preamplifier for a moving coil, stereophonic, pickup cartridge which interposes an essentially resistive electrical interface between each moving coil of that moving coil, stereophonic, pickup cartridge and the corresponding section of that preamplifier, and which makes the impedance of that electrical interface smaller than the impedance of the moving coil to which it is connected.

The amount of dynamic damping provided by the preamplifier of the present invention is small enough to enable the moving coil, stereophonic, pickup cartridge to retain a desirably high degree of compliance. As a result, the stylus of that cartridge will not unduly wear the walls of the microgrooves of the stereophonic disc with which it is used. This means that the preamplifier of the present invention enables the moving coil, stereophonic, pickup cartridge with which it is used to have a desirably high degree of compliance, and yet to have good trackability.

The preamplifier provided by the present invention can have the output thereof connected to the input of many different amplifiers for high fidelity systems. This is possible because that preamplifier interposes essentially resistive interfaces between the last stages ofamplification of the sections thereof and the corresponding inputs of the amplifier driven by that preamplifier. Those essentially resistive interfaces will dissipate some power; but the preamplifier of the present invention provides a total gain which equals the desired gain plus an additional amount of gain corresponding to the power which is dissipated in the essentially resistive interfaces. it is. therefore, an object of the present invention to provide a preamplifier for high fidelity systems which can have the output thereof connected to the inputs of various amplifiers, because it interposes essentially resistive interfaces between the last stages of amplification of the sections thereof and the inputs of those amplifiers.

Moving coil, stereophonic, pickup cartridges develop extremely low value output signalsone moving coil, stereophonic pickup cartridge that is presently being marketed producing an output signal having a nominal value of forty millionths of a volt. The manufacturer of that moving coil, stereophonic, pickup cartridge has recommended the connecting of the input terminals of a step-up transformer to the output terminals of that moving coil, stereophonic, pickup cartridge to develop a signal with an appreciably higher voltage. However, such step-up transformers tend to cause the signals at the output terminals thereof to have unacceptable amounts of hum and noise therein. In addition, where those step-up transformers are mounted within the moving coil. stereophonic, pickup cartridges, those transformers unduly increase the weights of those cartridges; and where those stepup transformers are spaced from those cartridges, the leads between those cartridges and those step-up transformers tend to pick up hum and transient noises. As a result, the use of step-up transformers with moving coil, stereophonic, pickup cartridges is not very satisfactory. The preamplifier of the present invention obviates all need of a step-up transformer, and thereby avoids the hum and noise which the use of such a transformer entails. In addition, that preamplifier utilizes inverse feedback to largely cancel out signals which are due to noise and transients; and the first stages of amplification of the sections of that preamplifier utilize transistors which have the emitters thereof directly grounded to minimize the generation of noise. As a result, that preamplifier is able to provide output signals which are essentially hum-free and are essentially distortion-free. It is, therefore, an object of the present invention to provide a preamplifier for moving coil, stereophonic, pickup cartridges which obviates all need of a step-up transformer, which utilizes inverse feedback to largely cancel out signals due to noise and transients, and which utilizes transistors in the first stages of amplification of the sections thereof that have the emitters thereof directly grounded to minimize the generation of noise.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description, a preferred embodiment of the present invention is shown and described, but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing:

FIG. 1 is a schematic diagram of one preferred embodiment of preamplifier that is made in accordance with the principles and teachings of the present invention;

FIG. 2 is a graph showing the frequency response of the preamplifier shown in FIG. 1;

FIG. 3 is a graph showing the frequency response of a stepup transformer which is used as a preamplifier for a moving coil, stereophonic, pickup cartridge, and

FIG. 4 is a graph showing the levels of the second harmonic distortion developed by the preamplifier shown in FIG. 1 and by the step-up transformer which is used as a preamplifier for the moving coil, stereophonic, pickup cartridge.

Referring to the drawing in detail, the numeral denotes one of the coils of a moving coil, stereophonic, pickup cartridge for a high fidelitysystem, and the numeral 120 denotes the other of the moving coils of that cartridge. The coil 20 will develop a signal corresponding to one of the stereophonic channels recorded on a stereophonic disc; and the coil 120 will develop a signal corresponding to the other of the stt reophonic channels recorded on that disc. The terminals 22 and 24 of the coil 20 are connectable to the

input terminals

26 and 28 of one section of a preamplifier 27; and the

terminals

122 and 124 are connectable to the

input terminals

126 and 128 of the other section 125 of that preamplifier.

Junctions

30 and 32 connect a low value resistor 34 across the

input terminals

26 and 28. The numeral 36 denotes an NPN transistor; and a capacitor 38 and a junction 40 connect the junction 30 to the base of thattransistor. The junction 32 is connected to the emitter of the transistor 36 and to ground by a junction 42. A resistor 44 has one terminal thereof connected to the base of the transistor 36 by the junction 40, and has the other terminal thereof connected to the collector of that transistor by a junction 46. A junction 48, a resistor 50, a junction 52, a resistor 56,

junctions

58 and 60, a resistor 63, a junction 64, and a single pole, single throw switch 66 selectively connect the collector of the transistor 36 to the positive terminal of a battery 68. The negative terminal of that battery is grounded. A capacitor 54 is connected between the junction 52 and ground; and a capacitor 62 is connected between the junction and ground.

A capacitor 70 and a junction 100 connect the junction 48 to the base of an NPN transistor 74. A resistor 76 is connected between the junction 72 and ground; and a resistor 78 is connected between the emitter of the transistor 74 and ground. A resistor 80 has one terminal thereof connected to the base of the transistor 74 by the junction 72, and it has the other terminal thereof connected to the collector of that transistor by a junction 82. The junction 82, a junction 84, and a resistor 86 coact with

junctions

58 and 60, resistor 63, junction 64 and the switch 66 to selectively connect the collector of the transistor 74 to the positive terminal of the battery 68. A capacitor 88 connects the junction 84 to the left-hand terminal of a T-pad which includes

resistors

90, 94 and 96 and a junction 92. A junction 98 connects the lower terminal of that T-pad to ground and to an output terminal 102; and the righthand terminal of that T-pad is connected to output terminal of the preamplifier 27. The

numerals

104 and 106 denote two of the input terminals of an amplifier 108 to which the

output terminals

100 and 102 of the preamplifier 27 will be connected.

Junctions and 132 connect a low value resistor 134 across the

input terminals

126 and 128 of the section 125 of the preamplifier 27. The numeral 136 denotes an NPN transistor; and a capacitor 138 and a junction 140 connect the junction 130 to the base of that transistor. The junction 132 is connected to the emitter of the transistor 136 and to ground by a junction 142. A resistor 144 has one terminal thereof connected to the base of the transistor 136 by the junction 140, and it has the other terminal thereof connected to the collector of that transistor by ajunction 146. A junction 148. a resistor 150, a junction 152, a resistor 156,

junctions

158 and 160, a resistor 163, the junction 64 and the switch 66 selectively connect the collector of the transistor 136 to the positive terminal of the battery 68. A capacitor 154 is connected between the junction 152 and ground; and a capacitor 162 is connnected between the junction and ground.

The numeral 174 denotes an NPN transistor; and a capacitor and a junction 172 connect the junction 148 to the base of that transistor. A resistor 176 is connected between the junction 172 and ground; and a resistor 178 is connected between the emitter of the transistor 174 and ground. A resistor has one terminal thereof connected to the base of the transistor 174 by the junction 172, and it has the other terminal thereof connected to the collector of that transistor by a junction 182. A junction 184 and a resistor 186 coact with

junctions

158 and 160, resistor 163, junction 64 and switch 66 to selectively connect the collector of transistor 174 to the positive terminal of the battery 68. A capacitor 188 connects the junction 184 to the left-hand terminal of a T-pad which consists of

resistors

190, 194 and 196 and a junction 192. The lower terminal of that T-pad is connected to ground and to an output terminal 202 by a junction 198; and the right-hand terminal of that T-pad is directly connected to the output terminal 200 of the preamplifier 27. The

numerals

204 and 206 denote two further terminals of the amplifier 108 to which the

output terminals

200 and 202 will be connected.

Although the components of the two sections of the preamplifier 27 could be. given different values, the following values have been found to be extremely useful and desirable:

Component Value Resistors

34 and 134 15

ohms Capacitors

38 and 138 I00

microfarads Transistors

36 and 136 2N i010

Resistors

44 and 144 390,000

ohms Resistors

50 and 150 5,600

ohms Capacitors

54 and 154 250 microfarads Resistors 56 and 156 47,000

ohms Capacitors

62 and 162 2,000

microfarads Resistors

63 and 163 680 ohms Battery 68 9 volts Capacitor 70 50

microfarads Transistors

74 and 174 2N1 306

Resistors

76 and 176 22,000 ohms Resistors 78 and 178 100

ohms Resistors

80 and 180 120,000

ohms Resistors

86 and 186 6,800

ohms Capacitors

88 and 188 50

microfarads Resistors

90 and 190 18,000 ohms Resistors 94 and 194 18,000 ohms Resistors 96 and 196 43,000 ohms The two

sections

25 and 125 of the preamplifier 27 shown in FIG. 1 are identical; both with regard to the configurations thereof and with regard to the values of the components thereof, and hence only one of those sections need be described in detail. Those sections of that preamplifier will be housed within the same overall casing, but a shield 210 of ferrous metal will be interposed between those sections to minimize cross talk between the channels of stereophonicallyrecorded information passing through those sections.

In the operation of the preamplifier 27 shown in FIG. 1. both of the moving

coils

20 and 120 of the moving coil. stereophonic, pickup cartridge will develop signals as the walls of the microgroove in a stereophonic disc move relative to the stylus of that moving coil, stereophonic, pick up cartridge. The terminals 22 and 24 will apply the signals developed by the moving coil 20 to the

input terminals

26 and 28 of section 25; and the

terminals

122 and 124 will apply the signals developed by the moving coil 120 to the

input terminals

126 and 128 of the section 125. The following description of the operation of section 25 of the preamplifier 27 can also be considered to be a description of the operation of section 125 of that preamplifier.

The signals which the moving coil 20 develops, as a microgroove in a stereophonic disc moves relative to the stylus of the moving coil, stereophonic, pickup cartridge, will be applied to the resistor 34 by terminals 22 and 24, input terminals 26 and 285 and

junctions

30 and 32. The resistor 34 constitutes an electrical interface between the moving coil 20 and the first stage of the section 25 of the preamplifier 27; and that resistor and that moving coil coact to constitute a closed loop, transducer-load system. in the preferred embodiment of preamplifier shown in FIG. 1, the impedance of the resistor 34 is l /bhms and thus is less than the 2 ohms output impedance of the moving coil 20; and hence that moving coil and that resistor constitute a closed loop, transducer-load system wherein the impedance of the load is less than the impedance of the transducer. As a result, the resistor 34 will coact with the moving coil 20 to provide dynamic damping of the stylusinduced movement of that moving coil. That dynamic damping is important; because it will permit the stylus of the moving coil, stereophonic, pickup cartridge to remain in engagement with both walls of the microgroove in a stereophonic disc almost continuously; and hence that resistor enables the moving coil, stereophonic, pickup cartridge to have a better transient response than it could have if it were connected to a stepup transformer. The impedance of the resistor 34 is, however, high enough to permit the moving coil, stereophonic, pickup cartridge to have sufficient compliance to keep the stylus of that moving coil, stereophonic, pickup cartridge from causing undue wearing of the microgroove in a stereophonic disc.

Whenever the switch 66 is closed, current will flow from the positive terminal of the battery 68 via that switch, junction 64, resistor 63,

junctions

60 and 58, resistor 56, junction 52, resistor 50,

junctions

48 and 46, resistor 44, junction 40, the base-emitter circuit of transistor 36, junction 42, and ground to the negative terminal of that battery; and that flow of current will render that transistor conductive. Also, current will flow from the positive terminal of battery 68 via switch 66, junction 64, resistor 63, junctions 60 and.58, resistor 86,

junctions

84 and 82, resistor 80, junction 72, the base-emitter circuit of transistor 74, resistor 78, and ground to the negative terminal of that battery; and that flow of current will render the transistor conductor 74 conductive. The 390,000 ohm value of the resistor 44 enables the transistor 36 to have a Q point which causes the first stage of amplification of the section 25 to saturate about 60 decibels above a normal input of 40 microvolts. That 0 point is not symmetrically located between the saturation and clipping levels of that first stage of amplification; but it makes the operation of that first stage of amplification about 6 decibels quieter than if a symmetrical 0 point was used.

The capacitor 38 and the junction 40 will couple any voltage, which the moving coil 20 develops across the resistor 34, to the base of the transistor 36. If the coil 20 develops a signal across the resistor 34 which provides a positive-going voltage at the junction 30, the capacitor 38 and the junction 40 will couple that voltage to the base of the transistor 36 and thereby render that transistor more conductive. However, if the coil 20 develops a signal across the resistor 34 which provides a negative-going voltage at the junction 30, the capacitor 38 and the junction 40 will couple that voltage to the base of the transistor 36 and thereby render that transistor less conductive. The input impedance of a 2Nl0l0 transistor connected in the common-emitter configuration is about 900 ohms and the capacity of the capacitor 38 is one hundred microfarads; and the resulting time constant of that capacitor and the input impedance of that transistor is long enough to provide good coupling and a good frequency response at frequencies as low as cycles per second.

The emitter of the transistor 36 is connected directly to ground by the junction 42, and that is desirable. By omitting an emitter resistor for the transistor 36, the present invention obviates the electrical noise which would arise from the thermal agitation that occurs in carbon resistors. Consequently, the first stage of amplification of the section 25 provides desirably quiet operation.

if the coil develops a signal across the resistor 34 which provides a positive-going voltage at the base of transistor 36, that transistor will become more conductive; and the resulting decrease in voltage drop across the collector-emitter circuit of that transistor will cause the voltage at the junction 48 to become less positive. The capacitor 70 and the junction 72 will couple the resulting negative-going voltage to the base of the transistor 74, with a consequent decrease in the conductivity of that transistor. The resulting increase in the voltage drop across the collector-emitter circuit of that transistor will develop a positive-going signal at the collector of that transistor, and thus at the junction 84. Because of the gain provided by the

transistors

36 and 74 of the two stages of amplification of the section 25 of the preamplifier 27, the positive-going signal at the junction 84 will be very much greater than the positive-going signal at the junction 30. The total gain of the two stages of amplification of the section 25 is greater than the gain which is needed to enable the output signal of that section to drive the amplifier 108; but that total gain is made high so the T-pad, which consists of

resistors

90, 94, and 96 and the junction 92, can be used as an essentially resistive interface between the second stage of amplification of the section 25 and the amplifier 108.

The use of the resistor 44 as a biasing resistor is desirable; because that resistor provides a small amount of inverse feedback from the collector to the base-emitter circuit of the transistor 36; and that inverse feedback will tend to attenuate any noise or transients reaching that transistor. Specifically, if any noise or transients develop a signal at the base of the transistor 36 which is positive-going in nature, the resulting increase in conductivity of that transistor will develop a negative-going signal at the collector of that transistor; and the resistor 44 will feed that negative-going signal back to the base of that transistor and thereby tend to cancel the positive-going signal developed by the noise and transients. The use of the resistor as a biasing resistor is desirable for the same reasonthat resistor providing a small amount of inverse feedback from the collectorto the base-emitter circuit of the transistor The resistor 50 has a relatively low value, and thus provides a relatively low output impedance for the first stage of amplification of the section 25 of the preamplifier 27. This is a further reason why that first stage of amplification provides desirably quiet operation.

The value of the resistor 76 is selected to provide a good match for the output impedance of the first stage of the section 25. The resistor 78 acts as an emitter resistor for the transistor 74; and that resistor will improve the thermal stability of the second stage of amplification of the section 25. That resistor will, because of the thermal agitation therein, introduce a certain amount of noise into the operation of the second stage of amplification of section 25; but the signal which is applied to the base of the transistor 74 has such a good signal-to-noise ratio that noise is not a serious problem in that second stage of amplification. The resistor 78 also provides additional inverse feedback because it does not have a capacitor connected in parallel with it, and thus helps improve the frequency response of that second stage of amplification. Specifically, if any noise or transients develop a positive-going signal at the base of the transistor 74, the resulting increase in conductivity of that transistor will make the emitter of that transistor more positive relative to the ground, and thus will provide a negative-going signal at the base of that transistor which will tend to cancel the positive-going signal developed by the noise or transients.

The resistor 56 and the capacitor 54 constitute a decoupling network which will keep noise, transients, and cross talk from reaching the transistor 36. That noise and those transients could come from the second stage of amplification of the section 25 or from the section 125, and that cross talk would come from the section 125. The resistor 56 also helps set the normal voltage at the collector of the transistor 36 at about Wyolts; and such a low collector voltage fosters quiet operation of the first stage of amplification of the section 25 of the preamplifier 27.

The capacitor 62 has a very low impedance to noise, transients and cross talk, and that capacitor is connected in parallel with the series-connected resistor 86, the collectoremitter circuit of transistor 74, and resistor 78. As a result, that capacitor largely bypasses to ground any noise, transients and cross talk, thereby providing good voltage regulation and good decoupling between the section 25 and the sections 125. The resistor 63 is a current-limiting resistor which protects the battery 68 against undue current drain during any periods, such as turn-on, when the capacitor 62 is not charged and thus essentially acts as a impedance.

The T-pad, which consists of

resistors

90, 94, and 96 and the junction 92, is an essentially resistive interface between the second stage of amplification of the section 25 and the amplifier 108; and that T-pad provides an eight decibel attenuation of all signals applied to it. That attenuation improves the si ,nal-to-noise ratio of the output signal; and the use of such an interface makes it possible for the section 25 to be coupled to almost any commercially acceptable, high fidelity amplifier.

As pointed out hereinbefore, the operation of the section 125 of the preamplifier 27 will be essentially identical to the operation of the section 25 of that preamplifier. This means that the resistor 134 will act as an essentially resistive interface between the moving coil 120 of the moving coil, stereophonic, pickup cartridge and the first stage of amplification of the section 12.5, and will coact with that moving coil to constitute a closed loop, transducer-load system wherein the impedance of the load is less than the impedance of the transducer. As a result, the resistor 134 will coact with the moving coil 120 to provide dynamic damping of the stylus-induced movement of that moving coil. That dynamic damping is important; because it will enable the stylus, to which the moving

coils

20 and 120 are attached, to faithfully follow the modulations on the walls of the microgroove in the stereophonic disc, even when the coil 120 senses stereophonicalIy-recorded signals which were recorded at high intensity levels.

The 390,000 ohm value of the resistor 144 enables the transistor 136 to have a 0 point which causes the first stage of amplification of the section 125 to saturate about 60 decibels above a normal input of 40 microvolts. That Q point is not symmetrically located between the saturation and clipping levels of that first stage of amplification; but makes the operation of that first stage of amplification about 6 decibels quieter than if a symmetrical Q point was used.

Because of the gain provided by the

transistors

136 and 174 of the two stages of amplification of the section 125 of the preamplifier 27, the total gain of those two stages of amplification is greater than the gain which is needed to enable the output of that section to drive the amplifier 108; but that total gain is made high so the T-pad, which consists of

resistors

190, 194 and 196 and the junction 192, can be used as an essentially resistive interface between the second stage of amplification of the section 125 and the amplifier 108. That T-pad provides and eight decibel attenuation of all signals applied to it, and that attenuation improves the signalto-noise ratio of the output signal of the section 125. The use of such an interface makes it possible for the section 125 to be coupled to almost any commercially acceptable, high fidelity amplifier.

By using the preamplifier 27 shown in FIG. 1, it is possible to greatly increase the trackability of a moving coil, stereophonic, pickup cartridge while keeping the compliance of that cartridge at a desirable high level. Where a step-up transformer has been used to amplify the signals from that moving coil stereophonic, pickup cartridge, and where a tracking force of 2 grans has been used. the stylus of that pickup cartridge has not faithfully followed the modulations of the walls of the microgrooves of stereophonic discs whenever that stylus sensed stereophonic modulations which were recorded at intensity levels in the range of l l centimeters per second. In contrast, where the preamplifier provided by the present invention has been used as the preamplifier for that same stereophonic pickup cartridge, the stylus of that stereophonic, pickup cartridge has been able to faithfully follow the modulations on the walls of the microgrooves of the stereophonic discs even when it sensed stereophonic modulations which were recorded at intensity levels in the range of twenty centimeters per second.

In addition, the frequency response of the preamplifier provided by the present invention is vastly superior to the frequency response of the step-up transformer which has been used to amplify the signals from moving coil, stereophonic, pickup cartridges. For example, FIG. 2 is a graph which was obtained by applying signals having the same amplitude but having individually-different frequencies, in the range of 20 cycles per second, to the input of the preamplifier shown in FIG. 1; and that graph shows that the output of that preamplifier was the same at all of those frequencies. In contrast, FIG.

3, which is a graph that was obtained by applying essentially 1 similar signals to a step-up transformer which has been used as a preamplifier for moving coil, stereophonic, pickup cartridges, shows that the output of that stepup transformer varied widely at some of those various frequencies.

It should also be noted that the second harmonic distortion of the preamplifier shown in FIG. I is very much smaller than the second harmonic distortion of the step-up transformer which has been used to amplify signals from moving coil, stereophonic, pickup cartridges. For example, the upper trace in FIG. 4 represents the second harmonic distortion of the step-up transformer which has been used to amplify signals from moving coil, stereophonic, pickup cartridge obtained from a stereophonic disc which had twenty-five hundred cycle per second tone bursts recorded on it at an intensity level of i0 centimeters per second; and the lower trace in FIG. 4 represents the second harmonic distortion of the preamplifier of FIG. 1 when that preamplifier was used to preamplify essentially similar signals. The second harmonic distortion of the step-up transfonner averaged as much as 4 16/ l00ths percent of the total output signal, whereas the second harmonic distortion of the preamplifier of FIG. 1 was very much lessbeing only 2 5/ IOths percent of the total output signal. The very appreciably lower second harmonic content in the output signals from the preamplifier of FIG. 1 enables that preamplifier to provide an extremely high fidelity preamplification of the signals which it receives from a moving coil, stereophonic, pickup cartridge.

The signal-to-noise ratio of signals at the output of the preamplifier of FIG. 1 is much better than the signal-to-noise ratio of signals at the output of a step-up transformer which is used to amplify signals from moving coil, stereophonic, pickup cartridges. Specifically, tests have shown that the signal-tonoise ratio of signals at the output of the preamplifier of FIG. 1 are 2 /ztimes better than the signal-to-noise ratios of signals at the output of such a step-up transformer. That better signalsto-noise ratio materially improves the fidelity of the signals which can be developed by any amplifier which is driven by the preamplifier of FIGJl.

The damping factor for the preamplifier of FIG. 1 is much higher than the damping factor for a step-up transformer which is used to amplify signals from moving coil stereophonic pickup cartridges. Specifically, the ths, damping factor for the preamplifier of FIG. 1 is about I 33/ l00whereas the damping factor for such a step-up transformer is only about 3/ l 00ths at best, and can be as low as l5/l00ths. In addition, because the damping factor for the preamplifier of FIG. I is insensitive to changes in frequency, that damping factor is much more desirable than the damping factor for such a step-up transformer, which is sensitive to changes in frequency.

stereophonic, pick up cartridge with which it is used to have a.

high degree of compliance and while having good trackability; and it provides an extremely good frequency response provides desirably low second harmonic distortion, and provides a desirably high signal-to-noise ratio. Consequently, the use of the preamplifier provided by the present invention makes it possible to attain a fidelity and a listening quality superior to that previously attained with any step-up transformer for a moving coil, stereophonic, pickup cartridge.

Whereas the drawing and accompanying description have shown and described a preferred embodiment of the present invention, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

lclaim:

1. A preamplifier for a moving coil stereophonic pick up cartridge having two moving coils, one'for each of two channels, which comprises:

a first channel preamplifier section connected to one of the moving coils of said moving coil stereophonic, pickup cartridge to receive and amplify signals generated by said one moving coil;

a second channel preamplifier section connected to the second moving coil of said moving coil stereophonic, pickup cartridge to receive and to amplify signals generated by said second moving coil;

a resistive electrical interface connected across the input of said first channel preamplifier section and connected across the output of the first said moving coil of said moving coil, stereophonic, pickup cartridge to coact with said moving coil to form a closed loop,- transducer-load system;

said resistive electrical interface having an impedance smaller than the impedance of the first said moving coil of said moving coil stereophonic, pick up cartridge, whereby said electrical interface and said moving coil form a closed loop transducer-load system wherein the impedance of the load is smaller than the'impedance of said transducer;

a second resistive electrical interface connected across the input of said second channel preamplifier section and connected across the output of said second moving coil of said moving coil, stereophonic, pick up cartridge to coact with said moving coil to form a closed loop, transducerload system;

said second resistive electric l interface having an impedance smaller than the impedance of said second coil, of said moving coil, stereophonic, pick up cartridge whereby said second electrical interface and said second moving coil form a closed loop, transducer-load system wherein the impedance of the load is smaller than the impedance of said transducer; and

whereby said electrical interfaces provide dynamic samping for said moving coils and the stylus connected to said moving coils.

section includes a transistor which has the emitter thereof directly connected to ground to obviate the thermal agitation developed within emitter resistors, and wherein the first stage of amplification of said second channel preamplifier section includes a transistor which as the emitter thereof directly connected to ground to obviate the thermal agitation develope within emitter resistors. 1

3. A preamplifier for a moving coil, stereophonic, pick up cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a transistor and a coupling capacitor which directly couples signals, developed across the first said electrical interface by saidpne moving coil. to the base of said transistor, and wherein said second channel preamplifier section includes a transistor and a coupling capacitor which directly couples signals, developed across said second electrical interface by said second moving coil, to the base of the second said transistor.

4. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a transistor with a biasing resistor connected between the collector and emitter thereof to provide inverse feedback and wherein said second channel preamplifier section includes a transistor with a biasing resistor connected between the collector and emitter thereof to provide inverse feedback.

5. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim. 1 wherein said first channel preamplifier section includes a transistor with a biasing resistor that is dimensioned to provide an unsymmetrical, low noise Q point for said transistor, and wherein said second channel preamplifier section includes a transistor with a biasing resistor that is dimensioned to provide an unsymmetrical, low noise Q point for said transistor.

6. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a plurality of stages of amplification, wherein said second channel preamplifier section includes a plurality of stages of amplification, wherein all of said plurality of stages of amplification of the first said section are 2. A preamplifier for a moving coil stereophonic, pick up w decoupled from said second section and wherein all of said plurality of stages of amplification of said second section are decoupled from the first said section.

7. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein the total gain of said first channel preamplifier section is greater than the gain required to develop a usable output signal for said first section, wherein the total gain of said second channel preamplifier section is greater than the gain required to develop a usable output signal for said second section, wherein a third resistive electrical interface is connected across the output of said first section and is connected across an input of an amplifier to be driven by said preamplifier, wherein a fourth resistive electrical interface is connected across the output of said second section and is connected across an input of an amplifier, wherein said total gain of said first section .is great enough to provide a usable output signal for said first section despite the signal attenuation caused by said third electrical interface, and wherein said total gain of said second section is great enough to provide a usable output signal for said second section despite the signal attenuation caused by said fourth electrical interface.

Claims

1. A preamplifier for a moving coil stereophonic pick up cartridge having two moving coils, one for each of two channels, which comprises: a first channel preamplifier section connected to one of the moving coils of said moving coil stereophonic, pickup cartridge to receive and amplify signals generated by said one moving coil; a second channel preamplifier section connected to the second moving coil of said moving coil stereophonic, pickup cartridge to receive and to amplify signals generated by said second moving coil; a resistive electrical interface connected across the input of said first channel preamplifier section and connected across the output of the first said moving coil of said moving coil, stereophonic, pickup cartridge to coact with said moving coil to form a closed loop, transducer-load system; said resistive electrical interface having an impedance smaller than the impedance of the first said moving coil of said moving coil stereophonic, pick up cartridge, whereby said electrical interface and said moving coil form a closed loop transducerload system wherein the impedance of the load is smaller than the impedance of said transducer; a second resistive electrical interface connected across the input of said second channel preamplifier section and connected across the output of said second moving coil of said moving coil, stereophonic, pick up cartridge to coact with said moving coil to form a closed loop, transducer-load system; said second resistive electrical interface having an impedance smaller than the impedance of said second coil, of said moving coil, stereophonic, pick up cartridge whereby said second electrical interface and said second moving coil form a closed loop, transducer-load system wherein the impedance of the load is smaller than the impedance of said transducer; and whereby said electrical interfaces provide dynamic samping for said moving coils and the stylus connected to said moving coils.

2. A preamplifier for a moving coil stereophonic, pick up cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a plurality of stages of amplification, wherein said second channel preamplifier section includes a plurality of stages of amplification, wherein the first stage of amplification of the first said channel preamplifier section includes a transistor which has the emitter thereof directly connected to ground to obviate the thermal agitation developed within emitter resistors, and wherein the first stage of amplification of said second channel preamplifier section includes a transistor which as the emitter thereof directly connected to ground to obviate the thermal agitation developed within emitter resistors.

3. A preamplifier for a moving coil, stereophonic, pick up cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a transistor and a coupling capacitor which directly couples signals, developed across the first said electrical interface by said one moving coil, to the base of said transistor, and wherein said second channel preamplifier section includes a transistor and a coupling capacitor which directly couples signals, developed across said second electrical interface by said second moving coil, to the base of the second said transistor.

5. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a transistor with a biasing resistor that is dimensioned to provide an unsymmetrical, low noise Q point for said transistor, and wherein said second channel preamplifier section includes a transistor with a biasing resistor that is dimensioned to provide an unsymmetrical, low noise Q point for said transistor.

6. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein said first channel preamplifier section includes a plurality of stages of amplification, wherein said second channel preamplifier section includes a plurality of stages of amplification, wherein all of said plurality of stages of amplification of the first said section are decoupled from said second section and wherein all of said plurality of stages of amplification of said second section are decoupled from the first said section.

7. A preamplifier for a moving coil, stereophonic, pickup cartridge as claimed in claim 1 wherein the total gain of said first channel preamplifier section is greater than the gain required to develop a usable output signal for said first section, wherein the total gain of said second channel preamplifier section is greater than the gain required to develop a usable output signal for said second section, wherein a third resistive electrical interface is connected across the output of said first section and is connected across an input of an amplifier to be driven by said preamplifier, wherein a fourth resistive electrical interface is connected across the output of said second section and is connected across an input of an amplifier, wherein said total gain of said first section is great enough to provide a usable output signal for said first section despite the signal attenuation caused by said third electrical interface, and wherein said total gain of said second section is great enough to provide a usable output signal for said second section despite the signal attenuation caused by said fourth electrical interface.